An experimental evaluation of thermosiphon solar water heaters with closed loop freeze protection

The purpose of this research was to examine the performances of two thermosiphon hot water heaters. One system ran a 50-50 ethylene glycol-water solution through a flat plate type collector and a copper coil heat exchanger placed inside the storage tank. The other system is conventional and ran the storage tank water through the collector with open pipes into and out of the storage tank (i.e. no heat exchanger). Obviously the heat exchanger system will perform less efficiently because of higher fluid viscosity and losses in the heat exchanger. But because this system will not freeze, it could be used in winter and doesn't need draining to prevent pipe freezing. The research centered on determining the exact extent of the decrease in efficiency of this type of system from the more conventional type of system. The results showed that the system with the heat exchanger performed, on the average, 9% less efficiently than the system without the heat exchanger. This figure is acceptable and such an ethylene glycol system is recommended for use in winter or when there is a possibility of freezing on cool nights from radiation losses.